1. Field of the Invention
This invention relates to water cooled nuclear reactors, and more particularly, to pressurized water reactors having in-core instrumentation (in-core instrument thimble assemblies) that enter the reactor vessel through penetrations from the top of the reactor vessel and are used to monitor the neutron activities and coolant temperature within the core fuel assemblies.
2. Description of Related Art
Many water cooled nuclear reactors utilize a core of vertically positioned fuel assemblies within a reactor vessel. To monitor the neutron activities and coolant temperature within the core fuel assemblies, movable in-core instrumentation, such as movable neutrons detectors, conventionally enter the core from penetrations in the bottom of the vessel. In a few instances in the past, leakage occurred at the penetrations at the bottom of the vessel which presented significant repair problems. Accordingly, it would be desirable to have all of the in-core instrumentation access the core through penetrations from the top of the reactor vessel. Additionally, fixed in-core neutron detectors have been employed that reside in the fuel assemblies during reactor operation. In addition to fixed in-core instrumentation that enter through penetrations in the bottom of the vessel, there are fixed in-core instrumentation that enter through penetrations in the top of the vessel. In this latter configuration, each in-core instrument thimble assembly is totally enclosed in a guide path composed of tubing. The lower portion of this guide path extends down into the fuel assembly. However, even the fixed in-core neutron detectors have to be withdrawn from the fuel assemblies before the reactor core can be accessed for refueling operations. Thus, it is therefore necessary to provide structure which can satisfactorily guide and protect the in-core instrumentation entering from the top of the vessel and mitigate the potential for leakage.
Guidance for the instrumentation is needed through the area above the upper core plate, which is just above the fuel assemblies, to an elevation above the upper support plate which is spaced from and sits above the upper core plate, so that the in-core instrumentation can be withdrawn so its lower most extremity is at least at or about the mid plane of the upper core plate. This is necessary so that the upper internals can be removed to access the core for servicing, such as refueling. The existing upper support columns are available in between the upper core plate and upper support plate assembly to provide such guidance. However, presently there is no support for the instrumentation above the upper support plate assembly through which the in-core instrumentation has to be withdrawn to clear the bottom of the upper core plate. Accordingly, a new structure is needed that will provide guidance and protection for the in-core instrumentation in an elevation above the upper support plate assembly without impeding coolant flow in the upper internals during reactor operation.